1. Field of the Invention
The invention relates generally to a cell module suitable as an electric power generation portion of a fuel cell, and, more specifically, to a cell module including a hollow-core electrolyte membrane. The invention also relates to a method for forming such cell module, and a fuel cell in which such cell module is used as a cell.
2. Description of the Related Art
In fuel cells, fuel and an oxidant are supplied to two respective electrodes electrically connected to each other, and the fuel is electrochemically oxidized, whereby chemical energy is directly converted into electric energy. Unlike thermal electric power generation, electric power generation by the fuel cells is not restricted by the Carnot cycle. Accordingly, the fuel cells exhibit high energy conversion efficiency. In polymer electrolyte fuel cells, polymer electrolyte membranes are used as electrolytes. Attention has been given to the polymer electrolyte fuel cells, particularly, as mobile electric power sources and electric power sources for movable bodies, due to their advantages that it is easy to downsize such polymer electrolyte fuel cells, the polymer electrolyte fuel cells operate at a low temperature, etc.
In the polymer electrolyte fuel cells, when hydrogen is used as fuel, the reaction indicated by the following equation (1) proceeds at the anode.H2→2H++2e−  (1)
The electrons generated by the reaction indicated by the equation (1) flow through an external circuit, work as electricity using an external load, and then reach the cathode. The protons generated by the reaction indicated by the equation (1) flow, while being hydrated with water, from the anode side to the cathode side through the polymer electrolyte membrane by electro-osmosis.
When oxygen is used as an oxidant, the reaction indicated by the equation (2) proceeds at the cathode.2H++(½)O2+2e−→H2O  (2)
The water generated at the cathode passes mainly through gas diffusion layers, and is then discharged to the outside of the fuel cell. As just described, the fuel cells are clean electric power generators that discharge only water.
Polymer electrolyte fuel cells including fuel cell stacks formed in the following manner have been mainly developed. Such fuel cell stacks are formed in the manner in which (i) a catalytic layer used as the anode is formed on one face of a flat polymer electrolyte membrane, and another catalytic layer used as the cathode is formed on the other face of the flat polymer electrolyte membrane, (ii) gas diffusion layers are formed on the respective sides of the flat membrane-electrode-assembly, and (iii) multiple flat cells, each of which is formed by arranging the membrane-electrode-assembly with the gas diffusion layers between flat separators, are stacked on top of each other.
Considerably thin proton-conducting polymer membranes are used as the polymer electrolyte membranes in order to increase the power density of the polymer electrolyte fuel cells. The proton-conducting polymer membranes having a thickness of equal to or less than 100 μm are mainly used. Even if thinner electrolyte membranes are used to further increase the power density, it is not possible to form cells that are drastically thinner than the commonly used cells. Similarly, the thickness of each of catalytic layers, gas diffusion layers, separators, etc, had been reduced. However, there is a limit to increases, due to reduction in the thickness of such members, in the power density per unit volume. For such reason, it may be difficult to meet the demand for more compact fuel cells.
In addition, there is a disadvantage that the production cost of fuel cells is high. Usually, sheet-like carbon materials having excellent corrosion resistance are used to form the separators. However, the carbon materials are expensive. In addition, grooves that serve as gas passages are usually microfabricated in the faces of the separators in order to substantially uniformly distribute fuel gas and oxidant gas to the entire faces of the flat membrane-electrode-assembly. Microfabricating such grooves makes the separators considerably expensive. This drastically increases the production cost of the fuel cells.
The above described fuel cells have many other disadvantages. For example, it is technically difficult to reliably provide sealing to the periphery of each of the multiple cells stacked on top of each other in order to prevent the fuel gas and the oxidant gas from leaking from the gas passages. Further, the electric power generation efficiency may be reduced due to deflection or deformation of the flat membrane-electrode-assemblies.
In recent years, polymer electrolyte fuel cells, in which a cell module formed by arranging electrodes on the inner side and the outer side, respectively, of a hollow-core electrolyte membrane is used as a basic unit of electric power generation, have been developed. Such technology is described, for example, in Japanese Patent Application Publication No. JP-A-09-223507 (Document 1), Japanese Patent Application Publication No. JP-A-2002-158015 (Document 2), Japanese Patent Application Publication No. JP-A-2002-260685 (Document 3), Japanese Patent Application Publication No. JP-A-2002-289220 (Document 4), and Japanese Patent Application Publication No. JP-A-2002-124273 (Document 5).
In the fuel cells including such hollow-core cell modules, members corresponding to the separators used in the flat cell modules are usually not required. In addition, the gas passages need not be formed, because different types of gases are supplied to the inner faces and the outer faces of the cell modules, respectively, to generate electric power. Accordingly, the production cost may be reduced. In addition, because the cell module has a three-dimensional shape, the specific surface area of the hollow-core cell module is greater than that of the flat cell module, which may increase the power density of electric power generation per unit volume.
It is considered that the electrode reaction occurs at a portion, at which an electrode catalyst contacts a proton-conducting substance and to which the reaction gas is supplied, namely, a three-phase interface. Accordingly, increasing the power density of the fuel cells by controlling the three-phase interface has been examined. However, under the present circumstances, it is difficult to appropriately design the three-phase interface. For example, the electrode catalysts may be buried in the proton-conducting substance, which interrupts a supply of the reaction gas. On the other hand, if the electrode catalysts are arranged at positions apart from the proton-conducting substance, a supply of protons from the proton-conducting substance or a supply of protons to the proton-conducting substance may be interrupted. Due to such inconveniences, the expensive catalysts made of noble metal are not effectively used.
Also, the electrons generated by the electrode reaction at the three-phase interface on the anode side pass through collecting member, reach the three-phase interface on the cathode side, and are used for the electrode reaction that occurs on the cathode side. Therefore, it is necessary to maintain good conduction of electricity between the electrolyte membrane and the collecting members. In order to provide conduction of electricity to the cell stack formed of the flat cells, usually, the cells are stacked on top of each other, and pressed to each other by applying relatively strong pressure. The applied pressure causes the membrane-electrode-assembly to closely contact the gas diffusion layers and the separators, thereby providing conduction of electricity.
The hollow-core cell modules lack the separators, which serve as the collecting members in the flat cells and which electrically connect the cells. Accordingly, the hollow-core cell modules require collecting members.
Document 1 describes using titanium (Ti) wires as the collecting members, and fitting the titanium wires to the electrodes that support platinum (Pt). Document 2 describes electrically connecting the ends of column-shaped electrochemical elements to each other by a conductive connecting pattern. Document 3 describes using external terminals connected to the catalytic layers as collecting members in tubular fuel cells. Document 4 describes connecting collecting electrodes to catalytic layers by lead wires. Document 5 describes installation of linear negative terminals and linear positive terminals.
As described above, it is difficult to perform design so that the three-phase interface is controlled to effectively use the electrode catalysts. In the hollow-core cell modules, it is also difficult to perform design so that the three-phase interface is controlled to effectively use the electrode catalysts.
Unlike the flat cell modules, in the hollow-core cell modules, it is difficult, due to the shape and the structure, to apply surface pressure for causing the electrodes and the collecting members to contact more closely. Accordingly, the conduction of electricity is likely to be insufficient.
When wires are used as the collecting members as described in the patent publications described above, the contact area between the electrodes and the collecting members is small. Accordingly, the conduction of electricity is more likely to be insufficient due to insufficient surface pressure.